Copper foil is used in the production of printed circuit boards. Although an excellent electronic conductor, there are problems inherent with the use of such foil. Copper is easily oxidized and corroded. In the production of printed circuit boards, it is generally necessary to bond the copper foil to a dielectric substrate to provide the foil with dimensional and structural stability. As plated or rolled, the adhesion of copper foil to such substrates is generally insufficient. Copper is also known to accelerate or catalyze the decomposition of dielectric substrates. For these reasons, copper foil is typically sold with one or more protective layers applied to its surface.
The current practice for applying adhesion enhancement and protective layers to copper foil typically involves the following sequence of steps. (1) A nodularized or dendritic copper layer is deposited on the foil surface. This dendritic layer can be applied to either the matte side or the shiny side of the foil, or to both sides of the foil. The dendritic layer is applied to roughen and thereby increase mechanical interlocking between the dielectric substrate and foil surface in order to increase the adhesion strength of the foil. (2) A barrier layer is then deposited on the dendritic layer of copper from step (1). This barrier layer is added to prevent thermal degradation of the metal-resin interface, thereby maintaining adhesion of the foil to the resin. (3) A stabilization layer of zinc and chromium is then applied to both sides of the foil. The stabilization layer aids in oxidation resistance, shelf-life and humidity durability. (4) A silane layer is applied over the stabilization layer to enhance adhesion and to improve humidity durability.
In copper foils having a metallic stabilization layer deposited from an electrolyte solution containing no hydrogen inhibitor, two weaknesses have been noted. The first such weakness is the problem of high temperature oxidation ("HTO"), which generally exists at some temperature for any copper foil. The stabilization layer is intended to reduce the HTO problem, but the benefit obtained is limited in known systems.
The second such weakness results from evolution of hydrogen at the metal surface during electrodeposition of the stabilization layer onto the metal surface. Hydrogen evolved at the surface causes local increases in pH, which in turn cause precipitation of dissolved species onto the metal surface, resulting in spots on the finished copper foil.
The prior art has failed to adequately address these weaknesses.